The invention relates to apparatus monitoring the deposition of a liquid-to-pasty medium on a substrate.
Such apparatus is described in the German patent document 4 217 736 C2. Therein each electrode is a sensor which is a component of a high-frequency oscillation circuit and as such detects a change in frequency when there is a change in the relative electric permittivity of the medium between the electrodes.
In this design the sensor is capacitive, that is it is inserted as a capacitor in the high-frequency oscillation circuit. Depending on the kind of medium between the two probes, that is, depending whether air is involved, or a substrate without a strip of glue or a substrate with a strip of glue of various thickness, the capacitance of such configuration will change. However the system capacitance strongly depends on the relative dielectric constant of the materials assuming much different values for air, glue and paper.
A typical change in system capacitance however also changes the frequency, allowing determining for instance whether the substrate comprises or not a strip of glue.
The known apparatus monitoring the deposition of a liquid-to-pasty medium on a substrate does its job well. However there may be malfunctions in some cases.
Based on the known apparatus of the German patent document 4 217 736 C2, it is the objective of the invention to create an apparatus that offers improved reliability and higher accuracy of measurement.
This problem is solved by the invention in accordance with which the apparatus measures the imaginary component of the permittivity of the substrate together with the medium between the two electrodes, the test electronics thereupon using this test value to determine the characteristic signal.
It is the insight of the invention that the permittivity, ie the dielectric constant, is a complex value, that is it comprises a real component and an imaginary component. Furthermore experiment has shown, with respect to the materials of significance herein, especially liquid-to-pasty glue such as is used in glue strips on cardboard, on paper mats or the like, that the imaginary components of the permittivity are larger, sometimes even by an order of magnitude, than the real components.
Based on such empirical findings, the invention concludes that, considering the numerically larger value of the imaginary component of the permittivity, measuring this imaginary component shall be simpler and more reliable when determining the nature of the tested material.
Derivation of the pertinent formulas is briefly discussed below. Further details can be found in the article xe2x80x9cFrequenz-Zugang der komplexen Permittivitxc3xa4txe2x80x9d, F H Duesseldorf [Germany] Labor Werkstoffkunde, Sep. 4, 1998, pp 1-14.
The individual microscopic effects noted when a dielectric material is situated in an alternating electric field are best stated by a complex permittivity
xcex5r=xcex5rxe2x80x2xe2x88x92jxcex5rxe2x80x3.
where xcex5rxe2x80x2 is the real component and xcex5rxe2x80x3 is the imaginary component of the permittivity xcex5r.
The particular microscopic phenomena affecting this value will be not be elucidated herein. Basically, they involve effects of alignment, ionic and electronic polarizations. The permittivity, and both its components, are strongly frequency-dependent.
The term xcex5rxe2x80x3 describes the dielectric losses and accordingly it is a measure of the energy absorbed by the glue.
These dielectric losses behave like ohmic heat losses. This fact can be expressed also by the so-called loss tangent
tan xcex4=xcex5rxe2x80x3/xcex5rxe2x80x2.
FIGS. 1 and 2 illustrate this matter. FIG. 1 schematically shows the equivalent circuit of an actual lossy capacitor. When applying an AC voltage U, a current I is set up in the capacitor. This current comprises two parts, namely the current Ic which would be set up in ideal capacitor, and parallel thereto the loss current Iv through a resistor, representing the dielectric losses as heat in the capacitor.
FIG. 2 is a diagram of the two components, namely lossy current and current through the idealized capacitor, which when added represent the total current I through the actual capacitor.
It follows from the equivalent circuit,
Y=G+jxcfx89C,
where Y is the admittance, G the dissipative conductance and jxcfx89C the reactive admittance in the loss-free capacitor.
In the event that a test object be present in the capacitor,
Y=jxcfx89*Cmaterial.
The capacitance of a parallel plate capacitor is given by the formula
C=xcex5oxcex5rA/d
where A is the surface of each plate of a parallel plate capacitor and d is the distance between these plates, xcex5r being the relative dielectric constant of the material.
The latter two formulas directly lead to
Y=jxcfx89*(xcex5rxe2x80x2xe2x88x92jxcex5rxe2x80x3)*xcex5oA/d.
Using herebelow
Co=xcex5oA/d
then
G+jxcfx89*C=jxcfx89*(xcex5rxe2x80x2xe2x88x92jxcex5rxe2x80x3)+Co
G+jxcfx89*C=xcfx89*xcex5rxe2x80x3*Co+jxcfx89*xcex5rxe2x80x2*Co
whence
C=xcex5rxe2x80x2*Co.
However this indicates that only the real component of the permittivity accounts for the capacitance. Accordingly the heretofore conventional capacitance measurements will not detect the permittivity""s complex component.
Attention is now drawn to Tables 1 and 2 below.
Table 1 shows a number of test values for paper inserted between the plates of a parallel plate capacitor. Table 2 shows the corresponding test values for the dielectric between the capacitor""s plates consisting of two paper layers sandwiching glue layers.
The notation used in these Tables is as follows: Cr is the capacitance portion of the tested sample""s capacitance taking into account the edge field of the parallel plate capacitor; Cs is the capacitance portion taking into account the stray field to ground; f is the applied frequency, C is the measured capacitance, Ck is the corrected measured capacitance, CLK is the corrected capacitance of the test system without a test sample, G is the admittance of the dielectric, CL is the capacitance of the test system without a test sample, xcex5rxe2x80x2 is the real component of the permittivity, xcex5rxe2x80x3 is the imaginary component of the permittivity, and tan xcex4 is the loss factor.
Further information also can be found in the aforementioned article by J Prochetta PhD.
It is immediately clear from the Tables that when the dielectric is glue, the absolute values of the imaginary component xcex5rxe2x80x3 of the permittivity are order(s) of magnitude larger than the real component xcex5rxe2x80x2. Consequently measuring this imaginary component of the permittivity will be far more revelatory about the dielectric situated between the two electrodes. This is the heart of the invention.
It is furthermore clear from the above that the ratio of glue xcex5rxe2x80x3 to paper xcex5rxe2x80x3 is more than two orders of magnitude larger than the ratio of glue xcex5rxe2x80x2 to paper xcex5rxe2x80x2.
In an advantageous implementation of the invention, the imaginary permittivity component is measured by testing the current, or a current drop, through the substrate. This procedure takes into account that when measuring the current, the dielectric""s imaginary permittivity component is especially easy to measure. From the above formulas, it follows
G=xcfx89*xcex5rxe2x80x3*Co.
This formula shows that the loss portion G is directly proportional to the imaginary permittivity component. Therefore measuring this lossy current at once provides the desired result.
In a further advantageous implementation of the invention, the measurement of the current or of the current drop is carried out using a current-controlled voltage amplifier, in particular using a current-to-voltage (I-U) converter. As a result minute currents can be measured in simple and advantageous manner.
In another advantageous design, the current-to-voltage converter is connected to an adder in turn connected to the output of a first operational amplifier. This configuration allows advantageous and simple further processing of the detected signal at the output of said current-to-voltage converter.
In another embodiment of the invention, a first phase shifter is mounted between the input of the first operational amplifier and an AC voltage source. The function of this first phase shifter is to compensate the current-voltage shift at the output of the current-to-voltage (I-U) converter.
In a further advantageous embodiment of the invention, the phase-shifter is phase-inverting. In this manner the apparatus can be adjusted in such a way that when adjusting an empty sensor, that is without substrate and without medium, the test result at the adder""s output shall be 0. In this manner the minute tested voltages can be advantageously processed when a dielectric shall be situated between the electrodes.
In a further advantageous embodiment of the invention, the current-to-voltage converter comprises a circuit having a third operational amplifier. This feature allows economic and simple manufacture of the current-to-voltage converter.
In a further advantageous embodiment of the invention, a first input of the third operational amplifierxe2x80x94in particular the inverting inputxe2x80x94is connected directly to one of the sensor electrodes. In this manner a signal can be detected without fear of interference.
In a further advantageous embodiment of the invention, the two electrodes are mounted on different sides of the substrate. This configuration is like a parallel plate capacitor with two planar electrodes between which the substratexe2x80x94with or without mediumxe2x80x94shall be moved. Furthermore this design allows measuring through the substrate and through the medium transversely to the plane of the substrate. This procedure makes possible very simple measurements.
Another alternative design is to configure the two electrodes on one side of the substrate. In this case the imaginary component of the permittivity also takes place between the electrodes, this time at least partly in a plane which is parallel to that of the substrate surface.